Abstract
The rapid developments in the field of digital aerial photogrammetry (DAP) in recent years have increased interest in the application of DAP data for extracting three-dimensional (3D) models of forest canopies. This technology, however, still requires further investigation to confirm its reliability in estimating forest attributes in complex forest conditions. The main purpose of this study was to evaluate the accuracy of tree height estimation based on a crown height model (CHM) generated from the difference between a DAP-derived digital surface model (DSM) and an airborne laser scanning (ALS)-derived digital terrain model (DTM). The tree heights determined based on the DAP-CHM were compared with ground-based measurements and heights obtained using ALS data only (ALS-CHM). Moreover, tree- and stand-related factors were examined to evaluate the potential influence on the obtained discrepancies between ALS- and DAP-derived heights. The obtained results indicate that the differences between the means of field-measured heights and DAP-derived heights were statistically significant. The root mean square error (RMSE) calculated in the comparison of field heights and DAP-derived heights was 1.68 m (7.34%). The results obtained for the CHM generated using only ALS data produced slightly lower errors, with RMSE = 1.25 m (5.46%) on average. Both ALS and DAP displayed the tendency to underestimate tree heights compared to those measured in the field; however, DAP produced a higher bias (1.26 m) than ALS (0.88 m). Nevertheless, DAP heights were highly correlated with the heights measured in the field (R2 = 0.95) and ALS-derived heights (R2 = 0.97). Tree species and height difference (the difference between the reference tree height and mean tree height in a sample plot) had the greatest influence on the differences between ALS- and DAP-derived heights. Our study confirms that a CHM computed based on the difference between a DAP-derived DSM and an ALS-derived DTM can be successfully used to measure the height of trees in the upper canopy layer.
Highlights
The implementation of sustainable forest management requires reliable, accurate and up-to-date information on forest ecosystems [1,2]
The analysis of average heights obtained based on airborne laser scanning (ALS) and digital aerial photogrammetry (DAP) data indicates that both remote sensing (RS) methods of tree height estimation tend to underestimate tree heights in comparison to the field-surveyed values; such a tendency was observed for each tree species
The results of our study confirm the statements presented by the authors of previous studies; notably, DAP-derived digital surface model (DSM) normalized using LiDAR-extracted digital terrain model (DTM) can be successfully applied in the estimation of tree heights in the upper canopy layer [38,47,70]
Summary
The implementation of sustainable forest management requires reliable, accurate and up-to-date information on forest ecosystems [1,2]. Information on forest resources plays a crucial role in decision-making processes and forest management planning, including at the strategic level (long-term planning) and in the case of operational forestry (silvicultural treatments, harvesting plans, etc.) [3]. Such information reflects the size, quantity and condition of forest resources and is provided as a result of forest inventories [4]. The high demand for accurate and up-to-date information and the constant goal of cost reduction have resulted in a growing interest in alternatives to ground-based forest inventory methods that are effective, precise and objective [6,7,8,9,10]. Tests performed in Nordic countries, such as Norway, Sweden and Finland, have verified that 3D data captured through ALS technology can be the main data source for forest management inventories (FMIs) [36,37]
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